Chemical trap efficiency of a construction site storm-water retention basin

Storm-water runoff from construction sites is an important source of nonpoint-source (NPS) pollution in urban areas, and retention basins are widely used on construction sites to control NPS pollution. Although design specifications for such basins may include sediment-trap efficiency requirements, little is known about how well these basins control chemical pollutants such as heavy metals and nutrients. These chemical pollutants exist in both particulate and dissolved forms in storm-water runoff and can change form during transport. Thus, runoff leaving a storm-water basin with little sediment still could contain significant chemical pollution. Data collected during storms for a retention basin in northern Ohio show that inflow and outflow sediment and pollutant loads often display similar temporal trends. For a storm event with a low (19.7%) sediment-trap efficiency (TE), lead, chromium, and cadmium had TEs of -0.4%, -55%, and -27.3%, respectively. The basin had 72.9% lead and 56.1% reactive phosphorus TEs for a second storm event with a high (88.6%) sediment TE, although the cadmium TE was still negative. TEs for reactive phosphorus associated with finer sediments and in dissolved form were about half the level of total reactive phosphorus. These results suggest that storm-water retention basins designed to prevent downstream pollution associated with sediments may not be as effective in removing chemical pollutants as they are in removing sediments. increased understanding of the complex properties and processes controlling this system of pollutants is required if practical methods are to be developed to improve the chemical TE of retention basins.